A semiconductor-employed switching device (transistor, thyristor, etc.) or rectifier device (diode) is widely used as a power inverter or converter circuit device. Under the present circumstances, a more compact device with lower losses is preferable for such semiconductor device for power application in order to meet future demands for higher power. While silicon has conventionally been used widely as a semiconductor material, wide band gap semiconductor materials having higher breakdown fields are being developed as next-generation semiconductor materials in light of the present circumstances. Since what is called wide band gap semiconductor materials such as SiC, group-III nitride semiconductor, etc. are expected to have low on-state resistance and high breakdown voltage for their material properties, significant size reduction and reduction in losses of a power controller are expected by constituting a semiconductor device for power application using these materials.
Requirements on properties for such power diode include: (1) small leakage current during reverse blocking; (2) high breakdown voltage during reverse blocking; (3) large output current at forward conduction; (4) short reverse recovery time at shutoff; (5) high peak surge current value; and the like. Off course, a diode made of a wide band gap semiconductor material is required to meet these requirements.
Conventionally practical, silicon-employed P-N junction diode and its modified P-i-N junction diode have a drawback of long reverse recovery time at shutoff because of the occurrence of carrier injection from both P and N sides, that is, the above requirement (4) is not satisfied.
On the other hand, a silicon-employed Schottky barrier junction diode is also in practical use. This diode has an advantage in that a reverse current at shutoff does not occur in principle, but has drawbacks of having large leakage current and low breakdown voltage at application of a reverse-bias voltage and low peak surge current. That is, the above requirements (1), (2) and (5) are not satisfied.
To improve the drawbacks of such silicon-employed diodes, a SiC-employed Schottky barrier diode has been developed and is publicly known (cf. “P-Type 4H and 6H-SiC High Voltage Schottky Barrier Diodes” R. Raghunathan and B. J. Baliga, IEEE ELECTRON DEVICE LETTERS, Vol. 19, pp. 71-73 (1998)).
A SiC-employed Schottky barrier diode as disclosed in the above paper achieves the effect of increasing the breakdown voltage unlike a silicon-employed one, however, the drawbacks of not meeting the requirements (1) and (5) have not been solved so far.
SiC single crystal includes many crystal defects (specifically, tubular voids, what is called micropipes) and thus disadvantageously makes it difficult to manufacture with stability a device of relatively large area that can ensure sufficient output current, resulting in poor yields in manufacturing process.
Further, since a P-N junction diode employing SiC causes carrier recombination resulting from such crystal defects, the output current is more likely to be limited, so that the above requirement (3) is not satisfied.
A Schottky barrier diode employing group-III nitride semiconductor instead of silicon or SiC has also difficulty in solving the aforementioned drawbacks of not meeting the requirements (1) and (5).
Further, manufacturing a P-N junction diode having p- and n-type layers made of group-III nitride semiconductor instead of silicon or SiC is not practical because of technical difficulty in manufacturing P-type group-III nitride semiconductor having a high hole density with stability.